Motor-driven hydraulic power steering apparatuses are conventionally known which are adapted to rotate an oil pump by an electric motor and supply a working oil to a power cylinder from the oil pump to mitigate an operation force required for operating a steering wheel.
Where the ambient temperature is extremely low (e.g., -40 .degree. C.), the working oil has an extremely high viscosity resistance at actuation of the electric motor.
In such a case, when the electric motor is actuated for steering assist, the electric motor cannot properly be driven due to a higher back pressure. Even if the driving of the electric motor is possible, the rotation speed of the electric motor cannot be increased to greater than a certain level, so that a fail-safe function is effectuated for protection of an electric motor driver element to deactuate the power steering function.
To avoid the deactuation of the power steering function, it is preferred to measure the temperature of the electric motor driver element to make a fail-safe judgment on the basis of the temperature.
Even if the power steering function is not deactuated, it takes some time before the working oil can be supplied to the power cylinder at a required flow rate. With the steering operation being started in such a state, the steering assist cannot sufficiently be performed, so that a so-called entrapped feeling is aggravated. The entrapped feeling may be mitigated by reduction in the viscosity resistance when the oil pump is driven to raise the temperature of the working oil. However, it takes some time before the temperature of the working oil rises, if the oil pump is driven at a low rotation speed. This causes a driver to feel discomfort for a while.